Re: [PATCH v2 RFC] userns: Convert xfs to use kuid/kgid where appropriate

Dave Chinner
2013-06-26T02:09:59Z

On Mon, Jun 24, 2013 at 09:10:35AM -0400, Dwight Engen wrote:
> Hi Dave, all. Here is a v2 patch that I believe addresses the previous
> comments, but I expect there to be more :) I think there are a few
> more issues to sort out before this is ready, and I want to add some
> tests to xfstests also.
>
> I added permission checks for eofblocks in the ioctl code, but
> I don't think they are enough. Just because an unprivileged
> caller is in a group doesn't mean he can write to a file of that group,
> and I don't know how we can check for that till we get the inode in
> hand. Brian, if you or anyone else could comment on how this should work
> for the regular user and write() ENOSPC cases that'd be great.
>
> The xfs code now uses inode->i_uid where possible instead of di_uid.
> The remaining uses of di_uid are where the inode is being setup,
> conversion to/from disk endianess, in dealing with quotas, and bulkstat.

Hi Dwight,

I haven't looked at the code in detail, I'll just mention that I
agree with Brain that we need to split this up into more patches.
The conversions are subtle and easy to misunderstand, so small
patches are good. I'd say at minimum separate patches are needed
for:

- permissions check on eof blocks
- splitting eof blocks structure (I made comments on that to
Brain's posted patch)
- introduce XFS conversion helpers
- implement VFS-XFS boundary conversion using helpers
- XFS ioctl conversions (maybe multiple patches)
- final patch to modify kconfig once everything is done

> We do need to decide on the di_uid that comes back from bulkstat.
> Right now it is returning on disk (== init_user_ns) uids. It looks to
> me like xfsrestore is using the normal vfs routines (chown, fchown,
> lchown) when restoring so that won't line up if the xfsrestore is run
> in !init_user_ns.

Right. This is a major problem because there's no guarantee that you
are running restore in the same namespace as dump was run in. If you
take the dump in the init_user_ns, then you can't restore it from
inside the namespace container, and vice versa.

> We could possibly convert to userns values
> before returning them from the kernel, but I doubt that will work
> well with the xfs quotas.

Well, quotas are already converted to/from userns specific values
before they get to XFS. Including project quotas, which I think at
this point is wrong. We have no test cases for it, though, so I
can't validate that it actually works as it is supposed to and that
we don't break it inadvertantly in the future.

[ I'm still waiting on Eric to provide any information or scripts
for how he tested this all worked when he did the conversions.... ]

> Should we just require that callers of bulkstat
> be in init_user_ns? Thoughts?

This is one of the reasons why I want Eric to give us some idea of
how this is supposed to work - exactly how is backup and restore
supposed to be managed on a shared filesystem that is segmented up
into multiple namespace containers? We can talk about the
implementation all we like, but none of us have a clue to the policy
decisions that users will make that we need to support. Until we
have a clear idea on what policies we are supposed to be supporting,
the implementation will be ambiguous and compromised.

e.g. If users are responsible for it, then bulkstat needs to filter based
on the current namespace. If management is responsible (i.e.
init_user_ns does backup/restore of ns-specific subtrees), then
bulkstat cannot filter and needs to reject calls from outside the
init_user_ns().

But if we have to allow both configurations - which I think we have to
because both cases are valid choices for a hosting provider to give
users - then how are we supposed to implement/handle this?

The same goes for the file handle interfaces - it's perfectly valid
for a user to run a userspace NFS server (e.g. ganesha) inside a
namespace container, but that will allow that process to provide
unchecked remote access to the entire underlying filesystem, not
just the namespace being exported. i.e. fs/export.c needs to be made
namespace aware in some way so that there is a kernel wide policy
for handle to file translations in namespace containers....

---

FWIW, one comment on the wrappers now that I've quickly browsed the
code:

> @@ -68,14 +68,15 @@ xfs_acl_from_disk(
>
> switch (acl_e->e_tag) {
> case ACL_USER:
> + acl_e->e_uid = xfs_kuid_from_disk(be32_to_cpu(ace->ae_id));
> + break;
> case ACL_GROUP:
> - acl_e->e_id = be32_to_cpu(ace->ae_id);
> + acl_e->e_gid = xfs_kgid_from_disk(be32_to_cpu(ace->ae_id));

I know I suggested it, but I have to say, that does look a little
weird. Normally the to/from disk routines do endian conversion
internally, so perhaps the conversion routines coul dbe better
named.

These:

acl_e->e_uid = xfs_uid_to_kuid(be32_to_cpu(ace->ae_id));
acl_e->e_gid = xfs_gid_to_kgid(be32_to_cpu(ace->ae_id));

I think read a whole lot better, and the endian conversion now makes
sense as that's converting the on-disk value first...

> @@ -101,7 +102,18 @@ xfs_acl_to_disk(struct xfs_acl *aclp, const struct posix_acl *acl)
> acl_e = &acl->a_entries[i];
>
> ace->ae_tag = cpu_to_be32(acl_e->e_tag);
> - ace->ae_id = cpu_to_be32(acl_e->e_id);
> + switch(acl_e->e_tag) {
> + case ACL_USER:
> + ace->ae_id = cpu_to_be32(xfs_kuid_to_disk(acl_e->e_uid));
> + break;
> + case ACL_GROUP:
> + ace->ae_id = cpu_to_be32(xfs_kgid_to_disk(acl_e->e_gid));

and:
ace->ae_id = cpu_to_be32(xfs_kuid_to_uid(acl_e->e_uid));
ace->ae_id = cpu_to_be32(xfs_kgid_to_gid(acl_e->e_uid));

Sorry for asking you to redo this - sometimes an idea I have doesn't
quite work out the first time :/

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com